Source for window setting channel

I am installing the side windows in my 105 Sprint GT (see photo), and am trying to figure out what goes between the glass and the channel. These are the windows on the two sides of the car, behind the roll-up windows.

The glass measures .180" thick, and the channel groove is .210-.215" wide, so it would seem that I would need rubber with a thickness of .015" (= 1/2 x (.210 - .180)) to fill in the space between the glass and channel. Several thicknesses of electrical tape might work, but I worry that it would become gummy and fall apart over time. I tried rubber from a bicycle inner tube, but it was way too thick (approx. .040").

Is .015" thick window setting material something that an automotive paint & supplies store might carry? Or, something that one of these generic restoration suppliers would stock (you know, the guys who sell fender welting for '39 Packards)? Or, will I need to go to a McMaster Carr?

Note: Yes, I know that the channel is attached to the body with 3 little bits of rubber + steel channel per side. I have that done. And I understand that you secured the glass to the channel before attaching the channel to the body - but, I'm doing it in the other sequence to ensure that I have the glass correctly positioned in the vertical plane.

ps - No, the attached photo isn't a double exposure (can you do that with a digital camera? Is the term "double exposure" about as dated as "dialing the phone"?) That's my Duetto reflected in the side glass

Thanks!